Optically dispersive elements are well known in the art. Prisms and diffraction gratings are typical dispersive elements. Due to their superior dispersion properties, diffraction gratings are often used in modern spectrophotometers. The prism achieves dispersion due to the difference in the material refractive index according to the wavelength. However, the diffraction grating uses the difference in diffraction direction for each wavelength due to interference. Operating principles of the optically dispersive elements are available from various sources. An example of a website covering this topic is http://www.shimadzu.com/an/uv/support/fundamentals/monochromators.html, the entire contents of which are incorporated herein by reference.
A spectrometer, or a spectrum sensor, is an instrument configured to analyze the spectral distribution of impinging light. The spectrometer differs from imagers or cameras in that the spectrometer does not generate a spatial map of incoming light or a two-dimensional image at one or more (typically three) wavelength bands, but generates a spectral image of the entire impinging light without regard to spatial distribution of the intensity of light. The spectral image includes a set of intensity measurements for each wavelength range, which is herein referred to as a spectral channel or a “channel.”
An “on-chip spectrometer” or a “spectrometer-on-chip” refers to a spectrometer employing a single chip on which semiconductor devices for measuring the intensity of light at a respective channel are mounted in parallel. The on-chip spectrometers differ from conventional spectrometers most prominently by the size. Typically, an on-chip spectrometer has a dimension less than 10 cm×10 cm×10 cm, such as less than 5 cm×5 cm×5 cm, and may weigh less than 200 g. This is a tremendous improvement in portability compared to conventional spectrometers, which typically have a dimension greater than 50 cm×50 cm×50 cm, and weigh at least 30 kg. The portability of the on-chip spectrometer is achieved by employing solid-state devices for each component of the spectrometer. Thus, a dispersive prism and a collimator in a conventional spectrometer is replaced by an array of band pass filters that allow passage of light only within the respective wavelength range, and an array of solid state detectors underlying the array of band pass filters. An example of such an on-chip spectrometer is described in U.S. Pat. No. 8,284,401 B2 to Choi et al. and U.S. Pat. No. 8,542,359 B2 to Choi et al, which are assigned to NanoLambda, Inc. as of 2017.
Optical spectroscopy allows complete characterization of the spectral distribution of light emanating from an object or an ambient. The information contained in the spectral distribution of light can be captured by a spectrometer, and can be used to detect and quantify the characteristics or concentration of a physical, chemical, or biological object. Spectroscopy is a non-destructive measurement. For example, optical diagnostics using spectroscopy allows acquisition of chemical and/or biological information without taking a physical specimen.
The sensitivity of each sensor pixel in an optical sensor or a spectrum sensor needs to be properly calibrated for an optical sensor or the spectrum sensor to function accurately. Calibrating each sensor pixel employing a single selected spectrum at a time is time-consuming and increases production cost. A method of employing a monochrometer to sequentially irradiate a single sensor with different incident wavelengths is known. In this case, a prism or a diffraction grating in the monochrometer rotates relative to the single sensor under calibration and only a small fraction of the light energy corresponding to the wavelength of choice at each moment is employed to calibrate the single sensor. Alternatively, a tunable light source emitting light with different wavelengths depending on the operational condition can be employed to calibrate a single sensor by varying the incident wavelength during the calibration process. Such methods can work in a lab environment, but do not provide sufficient throughput for mass manufacturing of spectrum sensors. Examples of websites covering this topic include: https://www.hamamatsu.com/resources/pdf/ssd/si_pd_kspd0001e.pdf for Si based Visual range, and https://www.hamamatsu.com/resources/pdf/ssd/infrared_kird9001e.pdf for IR range.